The road to reuse of walnut by-products: A comprehensive review of bioactive compounds, extraction and identification methods, biomedical and industrial applications

Walnuts have high nutrition value and are one of the most popular nuts worldwide. A large amount of by-products including green walnut husk, walnut shell, walnut septum and walnut meal are generated during walnut oil processing. Walnut by-products contain a variety of bioactive compounds that could be applied in food, chemical and biomedical industries as native additives and functional components. In this review, some recent advances in walnut by-products as potential sources of bioactive compounds are summarized. Special attention was given to main bioactive compounds in various parts of walnuts, such as their biological properties, extraction processes and applications in different fields. Walnut by-products are important source of functionally diverse bioactive compounds with antioxidant capacity, antibacterial, blood pressure regulating, immune enhancing, neuroprotective and memory consolidation. Moreover, walnut by-products are chemically inert, biodegradable, and non-toxic materials, and thus can be used in the preparation of biological activated carbon and filtering medium. Owing to these great application prospects, further studies on physicochemical properties, bioavailability and biosecurity should be considered. To save production cost, novel efficient extraction technologies of bioactive compounds from walnut by-products should be focused on in the food and drug industries.